Mollusca
Mollusca is the second-largest of animals. The members are known as molluscs or mollusks. Around 85,000 of molluscs are recognized. The number of fossil species is estimated between 60,000 and 100,000 additional species. The proportion of undescribed species is very high. Many taxa remain poorly studied. Molluscs are the largest phylum, comprising about 23% of all the named marine s. Numerous molluscs also live in and s. They are highly diverse, not just in size and structure, but also in behaviour and habitat. The phylum is typically divided into 8 or 9 , of which two are entirely . molluscs, such as , , and es, are among the most of all s—and either the or the is the largest known invertebrate species. The s ( s and s) are by far the most numerous molluscs and account for 80% of the total classified species. The three most universal features defining modern molluscs are a with a significant cavity used for breathing and , the presence of a (except for ), and the structure of the . Other than these common elements, molluscs express great morphological diversity, so many textbooks base their descriptions on a "hypothetical ancestral mollusc" (see image below). This has a single, " -like" on top, which is made of s and reinforced with , and is secreted by a mantle covering the whole upper surface. The underside of the animal consists of a single muscular "foot". Although molluscs are s, the tends to be small. The main body cavity is a through which circulates; as such, their s are mainly . The "generalized" mollusc's feeding system consists of a rasping "tongue", the radula, and a complex digestive system in which exuded and microscopic, muscle-powered "hairs" called play various important roles. The generalized mollusc has two paired , or three in s. The , in species that have one, encircles the . Most molluscs have , and all have sensors to detect chemicals, vibrations, and touch. The simplest type of molluscan relies on , but more complex variations occur. All produce , from which may emerge , more complex larvae, or miniature adults. The coelomic cavity is reduced. They have an open circulatory system and kidney-like organs for excretion. Good evidence exists for the appearance of gastropods, , and bivalves in the period, 541 to 485.4 million years ago. However, the evolutionary history both of molluscs' emergence from the ancestral and of their diversification into the well-known living and forms are still subjects of vigorous debate among scientists. Molluscs have been and still are an important food source for s. A risk of food poisoning exists from toxins that can accumulate in certain molluscs under specific conditions, however, and because of this, many countries have regulations to reduce this risk. Molluscs have, for centuries, also been the source of important luxury goods, notably s, , dye, and . Their shells have also been as in some preindustrial societies. Mollusc species can also represent hazards or pests for human activities. The bite of the is often fatal, and that of causes that can last over a month. Stings from a few species of large tropical s can also kill, but their sophisticated, though easily produced, venoms have become important tools in research. (also known as bilharzia, bilharziosis, or snail fever) is transmitted to humans by water snail hosts, and affects about 200 million people. Snails and slugs can also be serious agricultural pests, and accidental or deliberate introduction of some snail species into new environments has seriously damaged some s. Phylogenetic tree |1= s |2= s |2= ns ("limpet-like", "living fossils") |3= s ( s, s, s, s) |2= s ( s, , , etc.) |2= s (tusk shells) }} }} }} }} |2= ns (spicule-covered, worm-like) |2= ns (chitons) }} }} |2= s |1= '' |2='' '' }} |2='' '' }} |3='' '' }} }} }} Images File:HPIM1795.JPG|Cuttlefish of the order Sepiida File:Pinna noblis shell & byssus.JPG|'' Category:Tree of life